Modern day integrated chips contain millions of semiconductor devices. The semiconductor devices are electrically interconnected by way of back-end-of-the-line metallization layers that are formed above the devices on an integrated chip. A typical integrated chip comprises a plurality of back-end-of-the-line metallization layers including different sized metal wires vertically coupled together with metal contacts (e.g., vias).
Back-end-of-the-line metallization layers are often formed using a dual damascene process. In a dual damascene process, a dielectric material is deposited (e.g., low k dielectric, ultra low k dielectric) onto the surface of a semiconductor substrate. The dielectric material is then selectively etched to form cavities in the dielectric material for a via layer and for an adjoining metal layer. In a typical via-first dual damascene process, a via hole is first etched through two dielectric layers separated by an etch stop layer. A metal line trench is then formed on top of the via hole. After the via and trench are formed, a diffusion barrier layer and a seed layer are deposited within the cavities. An electro chemical platting process is then used to fill the via and metal trenches with metal (e.g., copper) at the same time. Finally, the surface of the substrate is planarized using a chemical mechanical polishing process to remove any excess metal.
The dual damascene process is subject to a number of potential etching defects that may affect the quality of the metallization layers. For example, to prevent a metal line trench from further etching the underlying via hole, a photoresist plug is inserted in the via hole before the metal line trench is etched. If the photoresist plug is formed to a height that is too high, the dielectric material may be under-etched leaving fence defects. In contrast, if the photoresist plug is formed to a height that is too low, the dielectric material may be over-etched leaving facet defects. Also, etching may damage the dielectric material during PR stripping (e.g., damage to the dielectric trench side-wall) or enhance the nodule shape (e.g., fringed metal line edges) caused by standing waves produced in the resist layer as a result of interference between incoming and the reflected light used in photolithography processes. Such etching defects may result in voids or pit defects that negatively affect the reliability of metal interconnect wires.